An ultrasound system has become an important and popular diagnostic tool since it has a wide range of applications. Specifically, due to its non-invasive and non-destructive nature, the ultrasound system has been extensively used in the medical profession. Modern high-performance ultrasound systems and techniques are commonly used to produce two or three-dimensional images of internal features of an object (e.g., human organs).
Generally, the ultrasound image is displayed in a Brightness-mode (B-mode) by using reflectivity caused by an acoustic impedance difference between the tissues of the target object. However, if the reflectivity of the target object is hardly different from those of the neighboring tissues such as tumor, cancer or the like, then it is not easy to recognize the target object in the B-mode image. Further, an ultrasound elastic imaging technology has been developed to display an image of the target object by using mechanical characteristics of the target object such as the elasticity of the target object. Such technology is very helpful for diagnosing lesions such as tumor and cancer. The tumor or cancer is relatively stiffer than the neighboring tissues. Thus, when stress is uniformly applied, a displacement of the tumor or cancer is typically smaller than those of the neighboring tissues.
The elasticity of a tissue is measured by using ultrasound data obtained before and after compressing the tissue. A pressure plate mounted on a front side of an ultrasound probe is used to compress the tissue. A user may press the pressure plate on the target object, for example, by using an ultrasound probe to thereby compress the tissues of the target object. In such a case, since strain in the tissues depends on the pressure applied by the user, the quality of an elastic image may be changed according to the pressure applied to the tissue. For example, if the pressure is relatively weak, then a difference in strain between the tumor or cancer tissue and the neighboring tissues thereof tends to be very small, while the tumor or cancer is hardly distinguishable from the neighboring tissues in the elastic image.
Further, if the pressure is relatively hard, then a correlation between the tumor or cancer tissue and the neighboring tissues is lowered, which results in deterioration of the quality of the elastic. Thus, an appropriate pressure is required to obtain an enhanced elastic image. Experimentally, when the strain of the tissues falls within a range of 1-3%, an optimal elastic image can be obtained.
Since the ultrasound elastic image is formed by obtaining ultrasound data in a frame unit, a pressing speed and a frame rate may be important factors in determining the quality of the elastic image. Assuming that the target object is a breast whose elastic image is commonly used, a thickness of the target object may be about 30 mm. When the target object is compressed by applying a pressure, a moving speed of a user's hand may be about 10 mm/s. Under the above conditions, an appropriate frame rate to obtain an average strain of 1% may be determined by the following equation (1).Frame rate>(10 mm/sec)/(0.3 mm/frame)=33 frames/sec  (1)
That is, when the frame rate becomes greater than 33 frames/sec, the average strain may be less than 1%. Otherwise, if the frame rate is lower than 33 frames/sec, then the strain may be increased over 1%, which may cause an error for determining a correlation between the lesion and neighboring tissues.
However, in case of obtaining ultrasound image data at a rate of at least 33 frames/sec, the number of scan lines for obtaining the ultrasound image data should be reduced. This may cause a problem since the quality of 2-dimensional black and white (BW) ultrasound image is lowered. Also, as the depth of the lesion becomes deeper, the frame rate may be lowered due to the physical characteristic of the ultrasound signals propagated into the target object. This also makes the quality of the ultrasound elastic image degraded. Further, to achieve the above frame rate, the user has to press the target object at the corresponding pressing speed.